"save as image" no confirmation when overwriting
There does not seem to be a way to enable any settings to cause a pop up confirmation dialog to appear. 

If you are not concentrated on what you are doing during the saving process, you can still automatically click through the dialog and save over if you save files a lot and get used to the process so some conscious effort is still needed in any case. If you find it hard to know, when you are going to override a file look out for red color. That is very easy to spot if you are looking out for it. The file name field turns red if the same file name exists: